The present invention relates generally to optical measurement systems. More specifically, the present invention relates to the measurement of features in semiconductor manufacturing.
The successful manufacture of advanced sub-micron sized semiconductor devices requires the detection, measurement and evaluation of defects and other features as small as 1 micron on the photographic mask (photomask) used to pattern the wafer. Feature inspection and measurement techniques for masks therefore play an important role in mask making and quality assurance.
Thus, it is becoming increasingly important to be able to identify and to correctly size mask defects, line widths, and other features that are under 1 micron in size. Accurate sizing of these features allows masks that are below specification to be repaired and prevents the needless and costly hold up of masks that do meet specification. One of the problems of assessing reticle quality at these sub-micron levels on an automatic inspection system, however, is that the size of these features cannot always be accurately, quickly and cost-effectively measured in a production environment.
It has long been known that mask inspection tools are not measurement tools and that the size information provided by these tools has limited value. Consequently, many mask makers have incorporated measurement aids at the inspection station or move the mask to a more suitable measurement tool in order to make classification decisions. Measurement aids used at the inspection station include calipers, grids, and software based video image markers such as gates, scales, grids, boxes and circles. These aids are fairly rapid, but ultimately require the operator to xe2x80x9ceyeballxe2x80x9d the boundaries of the defect. This activity is very subjective and can lead to an error in the measurement of the defect.
For example, feature size is often measured by measuring the distance between opposite edges of the feature. Once a feature is identified by an inspection machine, the operator uses a video microscope and a television camera to position a cursor on one side of the feature and another cursor on the other side of the feature. The operator must judge for himself the exact boundaries of the feature and must place the cursors where he sees fit. At this point, the operator pushes a button and the software blindly computes the distance between the two cursors in order to supply a rough approximation of the dimension of the feature. This measurement technique is operator dependent in that the operator must manually position the cursors on the boundaries of what the operator believes to be the feature. The operator may misjudge the type of a feature, its boundaries, or may simply misplace a cursor even if the feature is visible. The software then calculates the distance between the cursors, without regard for the type of feature, its true boundaries, etc. The above technique may be performed with a standard video microscope but is completely subject to the operator""s skill level and interpretation.
Alternatively, the mask may be removed from the automatic inspection tool and relocated on a more precise and repeatable measurement tool. However, this approach involves removing the mask from production, relocating the feature, and is thus impractical in a production environment. This technique is also costly, time-consuming and increases the handling risk. For example, an atomic force microscope (AFM) may be used to measure feature sizes; such a microscope is extremely accurate but is very slow, very expensive and is still subject to operator interpretation.
Another difficulty with light measurements of features less than 1 micron in size is that the wavelength of photons begins to interfere with the measurement of these 1 micron and less feature sizes. Many techniques do not adequately address the non-linearities associated with such measurements.
One approach that has been taken that uses calibration of an automatic inspection system in order to size defects is described in Characterization Of Defect Sizing On An Automatic Inspection Station, D. Stocker, B. Martin and J. Browne, Photomask Technology and Management (1993). One disadvantage with the approach taken in this paper is that it only provides a technique for measurement of defects of 1.5 microns and greater. Such sizes of defects would produce a linear relationship between reference sizes and actual measured sizes, and the paper does not account for defects less than 1 micron that would produce a non-linear relationship.
Of particular concern is the measurement of features used in Optical Proximity Correction (OPC). In general, photomasks are used to generate a desired pattern on silicon wafers using optical projection lithography. The optical projection causes blurring of the image created on the wafer surface. This blurring is especially noticeable on corners, and on edges near corners.
FIG. 1 illustrates the blurring effect on a corner during lithography. Design 10 represents a pattern for a silicon wafer that has a sharp corner. This design has been produced in a suitable computer program and is represented in a design database and is viewed by a user on a computer screen. Due to blurring during the process, mask 12 ends up having a slightly more rounded corner then the sharp corner in the design. When the mask is eventually used to produce wafer 14, the resultant corner is much more rounded, and is substantially different from the sharp corner desired in the original design.
FIG. 2 illustrates the blurring effect for the pattern of two line ends. Lines 20 and 22 in the original design are desired to be a certain distance apart. Due to blurring, lines 24 and 25 on the mask have more rounded ends and are slightly farther apart. The resultant wafer shows lines 26 and 27 having markedly rounded ends and being further apart then desired in the original design.
The OPC technique has been developed in order to correct for the detrimental affects of blurring in lithography. In most cases OPC corrections consist of xe2x80x9cserifsxe2x80x9d added to corners on the photomask. These serifs add or remove chrome at the mask corners in order to compensate for the area lost due to blurring. There is a large amount of literature dealing with the design of these OPC features. OPC features have become vital to the success of microlithography over the last few years. As chip geometries continue to shrink they will become yet more critical. Despite their widespread use, it has been very difficult to measure OPC features (called xe2x80x9cserifsxe2x80x9d) because their sizes are small, and by nature they appear as curves on masks and wafers. Conventional metrology tools are designed to measure the distance between edges. If the edges are irregular curves, as serifs are, edge-to-edge measurement becomes difficult, user dependent and sample dependent. Accurate measurement of these serifs as printed on photomasks (and then on wafers) is essential to the development of improved OPC algorithms and production processes.
FIG. 3 illustrates correction using Optical Proximity Correction (OPC). Using OPC, an additional shape 32 (termed a serif) is added to the corner of design 30. The resultant mask 34 from design 30 now includes a serif 36 which is more rounded in shape. Finally, the resultant wafer 38 has a fairly sharp corner which is the desired shape for the patent on the wafer.
FIG. 4 illustrates correction of blurring for two line ends using OPC. In the original design lines 40 and 42 are spaced a desirable distance apart and have serifs 41-44 added to each corner of each line ends. Line ends 46 and 47 on the mask produced from the design shows each line having more rounded serifs and the ends being slightly closer than desired. Finally, the resultant wafer shows a pattern of two line ends 48 and 49 having ever so slightly rounded corners and having the desired spacing between the two line ends.
One problem associated with OPC is that the added serifs may not be of the correct size. Serifs added to corners of a pattern may be either too large or too small, while serifs added to adjacent line ends may produce bridging (if the serifs are too large), or may produce line shortening (if the serifs are too small).
As with all manufacturing processes, variations from the design frequently occur. Some of those variations will cause a wafer produced from the photomask to fail. Photomask manufacturers need a method of measuring features in order to predict consistently which features will cause failures and which will not. This is especially needed for OPC features because the reason these features (usually serifs) are included in the mask design is because the position of the edge projected onto the wafer is critical at these positions.
As described above, the traditional method of measuring mask features, including OPC features, has been to measure the edge-to-edge size of features. This method becomes inaccurate when the size of the feature being measured approaches or becomes less than the wavelength of the light used to produce the image. In addition to becoming inaccurate, it also becomes noisy because the edge contrast diminishes with smaller features, thus any noise in the image will cause larger uncertainties in the edge position. Edge determination becomes more difficult when measuring OPC features because the edges are curved, thus it is important to measure the edge position carefully to get a meaningful measurement. A convenient technique to accurately measure OPC features at these small dimensions would be desirable.
To achieve the foregoing, and in accordance with the purpose of the present invention, an OPC feature measurement technique is disclosed that accurately measures serif area of OPC features, feature separation and line symmetry.
The flux-area measurement technique discussed in U.S. Pat. No. 5,966,677 has been shown to be accurate and consistent in predicting the size of defects and features printed onto wafers. The present invention discloses techniques for using flux-area measurement methods for measuring the relevant parameters of OPC features. The flux-area technique measures the parameter that affects printability, that is the amount of light flux that passes through a region of the mask and thus is available to activate the lithographic process. These measurements do not attempt to measure edge positions, thus they suffer from lower noise and less ambiguity due to edge curvature. Furthermore, because the flux-area technique measures light absorbed (or reflected), its measurements correctly reflect the presence of thin chrome, soft (dirt) defects, defects smaller than the resolution of the optical system and other features having ambiguous edges.
The effective area of a serif can be determined by measuring the area of defined regions of the feature, and then comparing these areas either to similar printed features, or to features simulated by extrapolating straight lines in the feature to be measured. Straight edges can be defined by a threshold value or by their intensity profile. The embodiments described herein are especially useful for measuring curved regions having a radius of less than about twice the wavelength (lambda) of the examining radiation, and for measuring distances between similarly curved features.
In a first embodiment, the relative area of serifs present on a line end is measured. A region of interest is defined around the line end and an intensity profile parallel to the line is created. The differences between the profile intensities in the serif area and a constant value (e.g., the average profile intensity on the shank of the line) are summed in order to calculate a serif area flux value. The flux value is divided by the intensity range to determine an area, which can then be converted into an effective diameter of a corresponding spot or hole. The effective diameter is used to quantify the total area of the OPC serifs.
In a second embodiment, the separation distance between a line end and another feature is determined. First, a region of interest that spans the separation distance between the two features is defined. Next, an intensity profile is created orthogonal to the line direction. The differences between data points on the profile and a constant value are summed to calculate a separation area flux value. The separation distance between the features is calculated from the separation area flux value and the height of the region of interest. The separation distance is useful for evaluating the quality of the photomask.
In a third embodiment, the asymmetry of serifs on a line end present on a photomask is determined. Two regions of interest are defined: a shank region of interest that includes the shank of the line and an end region of interest that includes the end of the line. Intensity profiles are created for both regions of interest. The profiles are created by summing values in a longitudinal direction with respect to the line. Finally, an asymmetry offset value for the serifs is calculated using the centroid of the end intensity profile and the centroid of the shank region profile. The asymmetry offset is useful for evaluating the quality of the photomask.